Please advice, How to handle it ?

Hi,

I am a Father of a 4 year old boy kid.

Need your opinion on how I should have handled a question from my kid.

Last weekend, We went for the shopping in a Mall. My wife was busy in shopping and I with my kid was wandering arround.

At last we got tired and found an empty chair in front of a Lingeri Shop.

There were two display (Female models) outside the shop.

One was wearing Sports wear, Tshirt and Short (Her Navel was not visible, only half of the stomach was visible)

and one was in undergarments. (Whose Navel was visble)

Suddenly my son spotted it and told that, Papa see - That girls Navel is not visible, where as in another we can see the Navel.

Then I told my son that - whenever we wear cloths our Navel gets hide beneath.

Then my son asked, if we will open the cloth then will it be visible ?

I told Yes, it would be.

Then he told, Papa I want to open and see.

Then I told , it would be no different , it would look like the other girl.

Then he told, I want to open that too and see.

I was stunned and laughed loudly.

I was not sure , what to reply him.

I just changed the topic and engaged him somewhere else and then he forgot.

But I couldn't forget that.

The main concern here is - His curiosity towards the female parts.

Is it normal for a kid to have curious for the opposite sex so early ?

Is it because, we take naked bath togather (Me and my son) and my wife conciously hides herself while changing dress or taking bath ?

May be he thinks that - I see papa daily but never seen Mamma ? (Can a 4 year old kid think like this ?)

Is my son behaving differently or all kids are same in this age ?

A Father from India.